A keratometer is a diagnostic instrument used for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It measures the central 3 mm of the cornea, which generally behaves like a sphere or a spherocylinder. A keratometer in relation to the cornea is just like an object in front of a convex reflecting mirror. Like in a convex reflecting surface, the image is located posterior to the cornea. The cornea behaves as a convex reflecting mirror and the mires of the keratometer are the objects. The radius of curvature of the cornea's anterior surface determines the size of the image.

Knee holders are used for holding a knee into selected positions for surgery. It is a base plate for mounting on an operating table, the base plate including a plurality of connecting elements extending along the length of the base plate and a leg brace for strapping to the lower leg of a patient.